In conventional manufacturing processes for the production of a final product from a molten material such as a metal, refractory or a glass, a melter such as a gas or electric furnace or a bushing is utilized. In order to maintain a proper product quality it is important to measure the temperature and/or mass flow rate or pour rate of the molten material emanating from the furnace.
Heretofore, the monitoring of these flow properties of a freely falling stream of molten material has been plagued with problems. For example, when measuring temperature, placing a contact temperature sensing device such as a thermocouple into the stream alters the flow of the material and may be rapidly eroded by a moving corrosive mass. When the moving mass is a solid object placing a device such as a thermocouple in contact with the moving mass results in rapid abrasive wear. Present non-invasive temperature sensing means such as optical pyrometers fail to provide any measurement of mass flow rates.
A so-called "catch bucket" or "ladle" method has been utilized to estimate mass flow rates. By this method a stream of molten material flowing from an over-head furnace is temporarily interrupted by inserting a ladle or a catch bucket of a known weight into the stream for a predetermined time period. The mass of the material that is accumulated during this period is then weighed and the mass per unit time calculated. This information may be utilized to effect a manual adjustment of the temperature of the furnace so that a desired mass flow rate may be obtained.
Although the "ladle" method of determining the pour rate of a furnace may have been appropriate for the manufacturing processes of the past, it suffers from a number of drawbacks. First, this method necessitates that the molten stream be intercepted thereby interrupting the flow rate of the stream and the continuity of any subsequent manufacturing steps. Secondly, the "ladle" method does not provide a continuous flow of data indicative of the mass flow rate of the furnace. Consequently, the "ladle" method provides representative data that is insufficient for a closed loop automated control of the furnace.
One prior are technique which provides a non-invasive method and apparatus for estimating the mass flow rate of a freely falling fluid stream from furnaces is disclosed in U.S. Pat. No. 4,090,241, issued May 16, 1978 to R. L. Houston. Houston uses an unmodified line scan camera to measure stream diameters of a molten glass stream at a plurality of locations. After the stream diameters are measured, a mass flow rate, proportional to the square of the measured widths is calculated.
It is unlikely that such a system would produce repeatible results due to several problems. These problems are: (1) fluctuations in brightness of the molten stream due to temperature variations across the width of the stream and with time; (2) edge effects surrounding the molten stream; (3) limitations in focusing the lens; (4) fringing effects within the camera itself and (5) smoke and other spurious images.
An additional problem in previous usage of an unmodified line scan camera which magnifies the problems, noted above, is that photodiodes within the camera, used to measure width, are usually "saturated", i.e., the diodes have been used near the upper limit where the output of a diode ceases to respond linearly to increases in the brightness or light intensity of the light sensed. This resulted from the recommended practice of insuring that sufficient light from the molten stream enters the camera and because the problems outlined above were not recognized.
Unfortunately, the cumulative effect of these problem phenomena precludes an accurate and repeatible determination of the fluid stream width. Inasmuch as a width is usually measured in a line scan camera by counting the number of diodes which register a predetermined voltage, it is essential that the correct number of diodes are counted. The usage of present unmodified cameras, without taking into account the problems alluded to above, insures that too many photodiodes register this voltage.
Consequently, the system disclosed by Houston is incapable of correctly ascertaining the mass flow rate of a stream of molten material. Additionally, the system of Houston does not measure the temperature of the molten stream emanating from the furnace.